1. Field of the Invention
The present invention relates to an engine control device for controlling the fuel injection amount of an engine mounted in an automobile or the like, and particularly to an engine control device having a failure judgment means to judge the failure of at least an intake pipe pressure sensor for detecting the measured intake pipe pressure of an engine.
2. Description of the Related Art
JP-A-2001-159574 proposes a diagnostic device for judging the failure of an intake pipe pressure sensor. This diagnostic device includes arithmetic means for calculating an estimated intake pipe pressure or an estimated intake pipe pressure equivalent value, an intake pipe pressure sensor for outputting a measured intake pipe pressure, and failure judgment means, in which the failure of the in take pipe pressure sensor is judged by comparing the estimated intake pipe pressure or the estimated intake pipe pressure equivalent value with the measured intake pipe pressure.
However, even if the failure diagnostic device disclosed in the prior art is adopted, in the case where a throttle valve opening degree is changed by an accelerator operation or the like of a driver of an automobile, even if the estimated intake pipe pressure is changed as indicated by a solid line in FIG. 7, the measured intake pipe pressure is changed later than the estimated intake pipe pressure as indicated by an alternate long and short dash line in FIG. 7. Thus, at the time of change of the throttle valve opening degree and for a transitional period of a specified time subsequent to this, a deviation occurs between the estimated intake pipe pressure and the measured intake pipe pressure. When the failure judgment of the intake pipe pressure sensor is performed in this transitional period, an erroneous judgment occurs, and therefore, it is necessary that the transitional period is made a failure judgment prohibition period.
Besides, in the calculation of the estimated intake pipe pressure, in the case where the calculation is performed on the premise that the atmospheric pressure is 1 atmosphere, as shown in FIG. 8, as the throttle valve opening degree is increased, the estimated intake pipe pressure rises up to 1 atmosphere. However, in a low atmospheric pressure state in which the atmospheric pressure is lower than 1 atmosphere at a high altitude or the like, as shown in FIG. 9, in a range where the throttle valve opening degree is a specified value or higher, the measured intake pipe pressure is kept at the lower atmospheric pressure. Thus, a deviation occurs between the measured intake pipe pressure and the estimated intake pipe pressure, and there is fear that an erroneous judgment of failure occurs even in this state. In order to prevent the erroneous judgment of failure, it is necessary that an atmospheric pressure sensor is provided to detect the atmospheric pressure, and the failure judgment is prohibited at the time of low atmospheric pressure.
Besides, in an engine control device using two throttle position sensors, that the throttle position sensor used for the failure judgment of an intake pipe pressure sensor is normal must be judged by a failure judgment function dedicated for the throttle position sensor. This increases a processing time of the engine control device and increases a memory capacity, and there is a disadvantage that the load of the engine control device becomes large.